1. Field of the Invention
One aspect of the present invention relates to a process for forming a photographic image using a silver halide light-sensitive material. More particularly, it relates to a process for forming a high-contrast negative image which simultaneously permits the production of dots possessing high image quality as is necessary in the field of graphic arts. Another aspect of the present invention relates to novel ethanedioic acid hydrazide compounds and compositions containing such compounds, said compounds and compositions being useful as dot-promoting agents in negative-working image systems.
2. Description of the Prior Art
In letterpress and offset lithography, tones cannot be reproduced by varying the amount of ink. A printing press can print only a solid color in the image areas, while leaving the non-image areas free of ink. In order to reproduce pictures in varying tones, graphic-arts photography uses a halftone screen. Halftone photography makes the printing of continuous-tone photographs possible by converting the continuous-tone image into a pattern of small and clearly defined dots ranging in size from 80 to 200 or more dots per inch according to the different amounts of light that are reflected from the different tones of the original.
All halftone dots have a fringe area surrounding them. When a printing plate is made of the halftone with a wide fringe area, the fringe around the dot is also partially exposed. Although it may not be apparent on the plate (i.e., the dot may appear to be of the correct size), once the plate is run on the press, the fringe area may eventually take ink which will fill in the shadow detail. Improvements in dot quality especially in regard to high-edge sharpness and minimal fringe (i.e., a "hard" dot) are therefore extremely important to the printing industry.
Traditionally, the production of high quality dot images was obtained with the use of "lith" films and chemistry. These films, used for making halftone or line images, were capable of producing extremely high contrast and good image sharpness. In the case of halftone images, such properties contribute to high "dot quality", i.e., the production of halftone dots of high density and sharpness. Sharpness is quantified in terms of "edge gradient" which is the ratio of change in density to distance at the boundary between the darkened part and the undarkened part of the photographic image. In general, the higher the edge gradient, the sharper the image (i.e. the "harder" the dot).
Those skilled in the art attribute the formation of hard dots produced with lith materials to the high contrast obtained from infectious development as described by Yule, J. Frank. Inst. 239 221 (1945). In fact, high contrast has come to be synonymous with high edge gradient. However, applicant has observed that a photographic element possessing high contrast is necessary but by no means sufficient to produce a hard dot. In addition, as Hirano (U.S. Pat. No. 4,429,036) states: "for use in the application of plate-making using a contact screen, such images having only the photographic characteristics of high contrast wherein the gradient is 10 or more are inferior in dot quality, are of too high contrast in screen range and therefore are not satisfactory." Thus, the high contrast of the element in and of itself may actually produce a screen range which is unacceptably short.
The photographic element must possess a wide screen range which allows good tone reproduction. This is obtained when the darkest area of the subject prints on the press as a solid and the lightest area prints with no evidence of a screen. The films may have very small unprintable dots in these areas which close up in the shadows and disappear in the highlights during printing; however, no loss of intermediate tones due to high contrast is allowable. Conversely, low contrast causes shadows to appear as 80-90% dots and 10-20% in the highlights; this, also, is unacceptable if a faithful reproduction is to be obtained.
Thus, although the dot quality derived from lith materials is excellent, the lith system suffers from serious deficiencies which restrict its utility.
As practitioners of the art recognize, the deficiencies of the lith system include a shortened useful life for the processing chemistry (due to the lack of sulfite) and a lower sensitivity for the lith photographic compositions (because it is necessary to use chloride or chlorobromide emulsions). Other problems inherent in the lith system include pepper spots, drag streaks, narrow screen range, and differences in sensitivity and gradation depending upon the manner in which materials are processed (tray vs. automatic processor).
Mifune (U.S. Pat. No. 4,323,643) discloses an alternative method for producing high quality dot images that solves some of the problems inherent in the lith system. The method involves the use of ureidophenylformyl hydrazide derivatives as additives to negative-working emulsions for the purpose of providing good dot quality. The mechanism by which the preferred agents operate is not mentioned. Mifune presents comparative data showing that various analogs (shown below in Table 1) do not produce acceptable dot quality as is required for the printing industry. Nevertheless, these same agents are reported in the prior art to produce high contrast negative images. ##STR2##
Many examples in the prior art disclose the use of hydrazine derivatives as contrast-promoting agents but the Mifune patent is the only one known to applicant in disclosing a select family of hydrazines that also produce good dot quality.
U.S. Pat. No. 2,419,975 teaches that high-contrast negative photographic characteristics can be obtained by adding a hydrazine compound to a silver halide photographic emulsion. The compounds listed in this patent are hydrazine derivatives that vary considerably in structure, but the quality of the screen dots obtained using these agents is not characterized. In general, large quantities of the disclosed agents are necessary to produce the desired high-contrast effect.
Recent patents teach that the most efficient hydrazines employ a combination of substituents to balance activity and stability. The stability of hydrazines was shown to be further increased by attaching directly to one of the nitrogen atoms a tertiary carbon atom (e.g., a carbon atom located in an aromatic ring). The art has long recognized that the activity of these stabilized hydrazines can be increased by the direct attachment of an acyl group to the remaining nitrogen. Presently, only a few substituents attached to the acyl carbonyl functionality have demonstrated utility in a negative-working, high-contrast system. These substituents include hydrogen (most preferred), unsubstituted alkyl (the activity of which dramatically falls as the chain length is increased) and, less desirably, aryl which is preferably substituted with electron-donating substituents (electron withdrawing substituents reduce the activity of the phenyl benzoic acid hydrazide). For example, Simson and Jordan (Canadian Patent No. 1,146,001 pp 15 line 16-24) state: "Although the hydrazine compounds intended for use in the practice of this invention each contain a formyl moiety, it is appreciated that otherwise comparable hydrazine compounds containing a benzoyl moiety substituted with a highly electron-withdrawing substituent such as a cyano group are operative. Such compounds have, however, been found to be inferior to the hydrazine compounds containing a formyl group."
Thus, the most commonly employed hydrazines are aryl formyl hydrazides. The more soluble agents of this class can be incorporated into the processing solution, but if they are to be incorporated into the photographic element, their mobility is preferably reduced. This can be achieved by incorporating either a ballast group or a functionality that promotes adsorption to the silver halide grain surface. The selection of an adsorption-promoting substituent for a phenyl hydrazide is limited in that "Tightly adsorbed aryl hydrazides are not usually efficient in increasing the contrast in negative-working silver halide emulsions. It is believed that contrast is increased by infectious development and that undue restriction of mobility interferes with the ability of the aryl hydrazides to promote infectious development" Parton, U.S. Pat. No. 4,459,347. The delicate balance necessary to provide adsorptivity to the silver halide grain while still providing adequate solubility, as well as the requirement for stability and inherent activity, place serious constraints upon the design of new aryl hydrazide contrast-enhancing agents.
When groups such as thiourea, thioamide, heterocyclic rings, or urea are used as adsorption-promoting functionalities, the molar concentration of the hydrazide can be reduced by an order of magnitude without loss of activity. This is a significant advantage over the use of mobile hydrazines because, at the high concentrations necessary to exhibit contrast enhancement in a negative emulsion, these mobile hydrazines release sufficient nitrogen to disrupt the ordered array of the photographic element and thereby deteriorate the image quality. Furthermore, diffusion of the mobile hydrazines into the processing chemistry alters the properties of the chemistry with time. Finally, the adsorption-promoting hydrazides are said to be less sensitive to the degree of stirring and temperature variation in the processing chemistry. This is a significant factor in reducing the differences in photographic speed and contrast found between tray and automatic developing for a given photographic emulsion.
Significantly, although both the mobile hydrazides and the adsorption-promoting hydrazides substantially increase the contrast of a photographic emulsion, only a select few of the latter class also improve dot quality. Undoubtedly, the dual constraints on controlled adsorptivity of the hydrazide and the printing parameters, which require tight control on screen range, severely limit the initially large number of choices of hydrazide derivatives that produce high contrast; it follows then that these constraints also limit the number of agents that can produce high quality dots since high contrast is a necessary factor in producing high-quality dots.
Despite the many advantages of photographic elements containing non-diffusable aryl hydrazides of the type found in U.S. Pat. No. 4,323,643, several defects still remain.
One such defect is that elements containing the nondiffusing hydrazides of the prior art have an even narrower screen range than that found in lith systems. This deficiency makes it increasingly difficult to accurately record all the detail in both the shadow and highlight areas of the continuous tone original. In general, the narrower the screen range, the harder (steeper) the dot gradation and hence the higher the contrast. Because the contrast of a silver halide emulsion containing an aryl hydrazide is dependent upon the particular aryl hydrazide, it would be most desirable to obtain contrast-enhancing agents producing a wider (softer) screen range.
Aryl hydrazides of the prior art have a tendency to produce dark spots on portions of the image that have not been (or have been only partially) exposed. These spots are known as "pepper grain". This phenomenon is observed as the concentration of the aryl hydrazide is increased and its onset generally coincides with the concentration necessary to produce contrast enhancement. As the processing chemistry becomes oxidized, on prolonged exposure to the air, pepper grain becomes more frequent and more pronounced.
A third and perhaps most serious effect associated with the use of the nondiffusing hydrazides of the prior art is their dependence on processing chemistry temperature. Attempts to solve this problem have included combining a nondiffusing hydrazide having a positive temperature dependency with other hydrazine derivatives that have a negative temperature dependency. U.S. Pat. No. 4,416,969 teaches the use of benzotriazole phenyl hydrazides in combination with thiourea-substituted phenyl hydrazides for the purpose of improving this temperature dependency.
In regard to the prior art relating to the present invention, Trivelli (U.S. Pat. No. 2,419,975) discusses the use of oxalyl hydrazide as a contrast-and speed-enhancing agent in a negative-working emulsion whereas Whitmore (U.S. Pat. No. 3,227,552) discusses the use of ethoxalyl-2-phenyl hydrazide in a direct-positive emulsion. Neither patent discloses the use of these agents for the production of high edge-quality dots. Considering Mifune's results with the thioureidophenyl formyl hydrazide, there would be no reason for those skilled in the art to expect that oxalyl hydrazides substituted with adsorption-promoting functionalities (such as thioureido) would afford any benefit or advantage in producing high edge quality dots.
It has now been unexpectedly discovered that oxalyl hydrazides substituted with a variety of substituents (e.g., thioureas, ureas, amides and heterocycles) attached to the phenyl ring impart excellent dot quality properties. Moreover, unlike the ureidophenyl formyl hydrazide series that apparently allows substitution on all sites of the phenyl ring, the compounds of the present invention require that the adsorption-promoting group be attached on the phenyl ring either ortho-or para- relative to the oxalyl hydrazide to produce the desired properties. Placement of an adsorption-promoting moiety (such as the thioureido group) either on the meta-position of the phenyl ring (compound II-3), directly onto the oxalyl moiety (compound II-5), or indirectly onto the oxalyl moiety (compound II-6) converts these compounds into desensitizing agents.